Quantum well (QW) lasers normally oscillate in the transverse electric (TE) polarization rather than the transverse magnetic (TM), because the n=1 heavy hole transition is the fundamental (lowest energy) transition, and the lowest energy state is the state whose population is most easily inverted. This is true for both unstrained GaAs and compressively strained InGaAs/AlGaAs and InGaAs/InGaAsP/InP QW lasers. The heavy hole is lowest in energy because the quantum shifts are inversely proportional to the effective mass; therefore the light hole levels are shifted to higher energies than the heavy hole levels. Additionally, in compressively strained InGaAs/AlGaAs and InGaAs/InGaAsP lasers, the strain leads to an even greater difference between these levels.
Transitions involving the heavy hole band have a 3:0 asymmetry for TE:TM mode gain, while those involving the light hole levels have a 1:4 asymmetry for TE:TM mode gain. Such a gain anisotropy does not occur in double heterostructure (DH) lasers, which have a bulk-like active region. This is why, in a QW laser, the TE-mode gain arises from the heavy hole transition, while the TM-mode gain is provided by the light hole transitions.
The first related application Ser. No. 07/948,531, whose contents are hereby incorporated by reference, discloses the implementation of monolithic, multiple beam laser diode sources for use in multistation xerographic processors with single optics. The devices in this implementation could produce laser beams with two nearly orthogonal polarizations. A quadbeam laser diode device implemented with this concept would permit considerable simplification in a four-beam optical system through the use of beam separation by polarization at two wavelengths. The quadbeam laser device is achieved by appropriate orientation of closely spaced laser chips within a single integrated package or by the relative orientation of the linearly polarized beams emitted by a monolithic laser array. Such monolithic arrays are obtained, for example, by combining the multiple wavelength techniques disclosed in U.S. Pat. No. 5,048,040 issued to Paoli with the multiple polarization techniques disclosed herein.